Nanowires are wire-like structures on the order of about 1 to 100 nm in diameter with a length of about 1 to 30 μm. Nanowires offer unique opportunities in studying electronic transport in low-dimension structures. A very high surface-to-volume ratio also makes nanowires attractive as adsorption-based chemical and biological sensors.
Single-crystalline one-dimensional semiconductor nanostructures are considered to be one of the critical building blocks for nanoscale optoelectronics. Control over the nanowire growth direction is extremely desirable, especially because anisotropic parameters such as thermal and electrical conductivity, index of refraction, piezoelectric polarization and band gap may be used to tune the physical properties of nanowires made from a given material.
Gallium nitride is a wide-bandgap semiconductor and a prime candidate for use in high-performance, high power optoelectronic devices because of its high melting point, carrier mobility, and electrical breakdown field. Single-crystalline gallium nitride planar structures and nanowires have shown promise for use in photonic and biological nanodevices such as blue light-emitting diodes, short-wavelength ultraviolet nanolasers, and nanofluidic biochemical sensors.
Furthermore, bottom-up fabrication of semiconductor nanostructure is of considerable interest for exploring quantum phenomena and future-generation devices. Employment of vapor-liquid-solid (VLS) mechanism of growth renders a unique pathway to one dimensional (1D) semiconductor nanowires that are promising components in nanoscale technology. Based on the VLS principle wide bandgap GaN nanowires have been prepared using techniques such as pulsed laser deposition, near-equilibrium tube-furnace deposition, and, most recently, conventional metalorganic chemical vapor deposition (MOCVD).
In VLS growth, a liquid metal cluster or catalyst acts as the energetically favored site for the absorption of gas-phase reactants. The cluster supersaturates and grows a 1D structure of the material. The VLS mechanism can be divided into three main stages: 1) nucleation, 2) precipitation, and 3) deposition. Prerequisites to VLS growth that have been identified include: (1) a sizable disparity in reaction kinetics between regular vapor-solid (VS) and the VLS mechanisms, thus mandating a lower supersaturation for growth selectivity; (2) the creation and retention of liquid droplets to facilitate adsorption and incorporation of vapor phase species; and (3) the need to have nucleation sites with appropriate crystallographic orientations conducive to the minimization of surface energies. Maintaining the VLS growth hinges on the identification and retaining of stoichiometry due to the strong tendency of GaN to solidify under a nitrogen-rich growth condition.
In contemporary synthesis of semiconductor nanowires, substrates have traditionally played a passive role in providing mechanical support and statistically averaged nucleation sites. A variety of substrates, including alumina, quartz, silicon dioxide, silicon (111), and C-axis sapphire have been explored. In almost all cases, morphology of haystack-like, random-oriented GaN nanowire is observed. In order to facilitate further processing and characterization, sonication of nanowires in organic solvents is performed to form a suspended solution. Nanowires are then transferred in solvent solution and become weakly attached (through van der Waals force) to the surface of target substrates during the evaporation of solvent. Elaborate techniques such as field-assisted alignment, micro-fluidic positioning, and the Langmuir-Blodget technique are then used to control the spatial distribution and ordering of the individual nanowires prior to contact lithography. Because of the challenges related to the current post-synthesis micro-scale manipulation of nanowires, it is highly desirable to provide an improved technique for more consistent and orderly growth of nanowires that is capable of overcoming some of the difficulties of the prior art.